In a core and metro optical communication network, large-capacity transmission with a rate exceeding 100 Gbps, that is, over-100 Gbps class transmission is required per optical fiber. Examples of a technique to implement over-100 Gbps class transmission include super-channel transmission in which a number of carrier waves called subcarriers are arranged in an extremely high density to transmit signals. By the super-channel transmission, usage efficiency of optical frequency is enhanced, and capacity enlargement is achieved. Meanwhile, in the super-channel transmission, interference occurs between adjacent subcarriers, and signal quality is degraded, since subcarriers are arranged with a high density on optical frequency.
In addition, in super-channel transmission in a core and metro optical communication network, optical filters for wavelength selection are arranged in a transmission path at multiple stages, and demultiplexing, multiplexing, path switching, and the like for a desired number of subcarriers. Consequently, in super-channel transmission, a subcarrier signal band other than a transmission band of the optical filters is removed, that is, signal quality is degraded due to a signal band being narrowed. Furthermore, in super-channel transmission, the optical filter transmission band becomes narrow depending on the number of optical filter stages to pass through, and also signal quality further worsens accordingly.
To suppress prevent signal quality degradation caused by interference between subcarriers and signal band narrowing in super-channel transmission in a core and metro optical communication network, it is necessary to determine an optical frequency band of a super-channel signal containing all subcarriers such that signal quality degradation does not occur. However, the optical filter transmission band is characterized in that the band discretely changes like in 12.5 GHz, for example, and thus a surplus band occurs in the optical frequency band of a super-channel signal depending on the number of optical filters. To address such a problem, Patent Literature 1 discloses a technique to effectively use an optical frequency band by arranging super-channel signals with narrowing the distance between the signals, in order to prevent occurrence of a surplus band between the super-channel signals.